Investigating the impact of seismic surveys on threatened sea snakes in Australia's North West Shelf

A project undertaken at the School of Earth and Environmental Sciences, The University of Adelaide, and supervised by Dr Kate Sanders

Summary

During off-shore oil and gas exploration, seismic surveys use air guns to direct intense impulses of sound at the sea floor, demonstrably causing physiological damage and disruptive behaviour in diverse marine taxa. Impacts of seismic surveys have not yet been assessed for sea snakes, despite dramatic population declines in the northwest - a region of intensive exploration. Several morphological and behavioural characteristics suggest sea snakes might be negatively impacted by air gun use. This project aims to combine captive and field experiments to determine whether air gun sound produced during seismic surveys influences behaviour, performance or health status of sea snakes.

Key outcomes

Behaviour. Field experiments were trialled over 10 days in the Ningaloo Marine Park, Western Australia, August 2013. We initially deployed Baited Remote Underwater Video Systems (BRUVS), equipped with underwater speakers, to assess impacts of airgun sound on sea snake behaviour. BRUVS recorded very few sea snakes, so we trialled an alternative method that involved actively searching for snakes and using a baited monopod with a GoPro attached at a fixed distance from the underwater speaker. The aim was to test for correlation between the time for change in underwater sound and time for change in snake behaviour. We were able to perform this experiment on six olive sea snakes (Aipysurus laevis). None of the snakes showed an observable change in behaviour at the initiation of (or during) the sound treatment. We used a powerful underwater speaker (Clark Synthesis AQ339) to expose snakes to a peak sound pressure of 66.3 db~µPA at 1 metre with dominant frequencies between 20 and 100 Hz. However, although startle responses were seen in nearby fishes, the sound generated did not reach the received levels considered harmful for other marine vertebrates (above 100 db re µPA). Due to these technical difficulties in triggering reactions of wild sea snakes to underwater sound, we focused the remainder of the project on morphological and electrophysiological approaches. Future studies will be needed to examine the behavioural and physiological effects of sounds (ideally using real airgun sources) on sea snakes.

Morphology. Scanning electron microscopy and comparative phylogenetic analyses were used to provide evidence that the scale sensilla (touch receptors) of terrestrial elapid snakes may function as hydrodynamic receptors in sea snakes. Scale sensilla were more protruding (dome-shaped) in sea snakes than in their terrestrial counterparts, and exceptionally high overall coverage of sensilla was found only in the sea snakes. High sensilla coverage appears to have evolved multiple times within sea snakes, so that the impacts of anthropogenic noise on sea snakes will likely vary among species. These findings are now published (Crowe-Riddell et al. 2016 Open Biology, 6(6):160054-1-160054-12) and were used to inform taxon selection in the electrophysiology study (below).

Electrophysiology. Auditory evoked potentials (AEP) of wild caught sea snakes were measured in 2015 and 2016, providing the first experimental data on the hearing abilities of sea snakes underwater. The audiogram of Hydrophis stokesii (based on two individuals) shows a limited frequency range of about 40 Hz to about 1000 Hz, peaking at low frequencies (60 Hz). This sensitivity is similar to species of fish only receptive to particle motion (e.g. fish without a swim bladder, elasmobranchs), which could suggest that sea snakes are not sensitive to sound pressure. By overlapping the signature of a typical airgun on the audiogram of H. stokesii, we predict that these snakes are able to detect an airgun sound up to 100 m from the source. We are currently preparing these results for publication.